I think it is far more likely that a major meteor strike(s) destroyed the ozone layer and they died of skin cancer. Probably the reason that alligators which hide in hollows under mud banks, feathered birds, smail hairy mammels, etc. survived in sufficient quantities to replenish the earth.
...Casadevall, of Albert Einstein College of New York... has long been troubled by the lives of warm-blooded animals, who must live a virtual food-finding mission because they burn so many calories each day just heating their bodies. Cold-blooded animals, on the other hand, need only eat once every few days... Fungal infections rarely give mammals more than a mildly irritating case of athlete's foot or a yeast infection but are often deadly to plants, fish, and insects... Researchers last year discovered fossil evidence of a post-collision "fungal spike," and in a world dense with potentially pathogenic fungi, warm-blooded animals might have had a unique advantage.I'd forgotten about this, rediscovered it tonight, pretty interesting. :')